Blown 96 WS-6

I'm no longer holding fuel pressure after the pump shuts off. It comes up quickly then immediately drops to zero when the pump shuts off. I've eliminated the AFPR and injectors as sources of the leak down leaving a faulty check valve in the Walbro as the likely suspect.

My question is: where, in the pump, is the check valve located? I ask because I'm considering putting a check valve in the line outside of the tank and I don't know if the check valve would cause a restriction issue. If the pump's built in valve is also located on the "discharge side" then it should be a wash. No?

Also, anyone know of a valve that would do this and would there be any issue of air in the line between the "new" check valve and the pump? My suspicion is that the fuel between the new valve and the pump would not drain out because there would be no way for air to get in to the system. Thinking of the scenario where you put a drinking straw into a glass of water then before withdrawing it you put your finger over the top. When you pull the straw out, the water remains until you take your finger off. is this logical?

I really don't want to drop the tank again. I put this pump in just a year ago